Foods such as fresh, precut vegetables as a product in a package formed from film present unique problems. When living as part of a plant, vegetables absorb carbon dioxide from the atmosphere and release oxygen. However, when cut or picked from the plant, the vegetable tends to absorb oxygen and release carbon dioxide (reverse photosynthesis). Thus, the nature of vegetables calls for packaging having certain permeability characteristics.
From health, safety and shelf-life perspectives, packaging for fresh, precut vegetables must have certain gas permeability characteristics. For instance, packaging for a leafy vegetable such as chopped cabbage without dressing (precut cole slaw) must have an oxygen transmission rate (OTR) of about 300 to 400 cc/100 in.sup.2 /24 hr. If precut vegetables are in a package which has low permeability to gases such as oxygen (OTR), bacteria growth inside the package can be stimulated, which can spoil the vegetables. The bacteria can also grow undetected without malodor or spoiling the food, and cause sickness. Further, this condition can lower the shelf-life of the vegetables. An approach to obtaining desired carbon dioxide and oxygen transmission through a package is to provide the package with loose or open seals or, perforations. However, this approach literally leaves the food exposed to all possible contaminants, including bacteria, lowers the shelf-life of the food, and, to some extent, defeats the purpose of employing packaging.
In addition, food such as fresh precut vegetables is an item which the consumer prefers to visually inspect prior to purchasing, for instance, to see if the vegetables have spoiled. If the packaging is cloudy or not sufficiently transparent or translucent, the food is not especially desirable to the consumer. Likewise, the packaging should not be too soft in order to provide adequate protection for the food. The packaging should also be able to be sealed, such as by heat-sealing, with a seal which is sufficiently strong so as to also provide adequate protection for the food. Further, the packaging should not be constructed from materials having high levels of extractables because these materials tend to impart unpleasant odors or tastes to the food.
To meet the unique situation of vegetable packaging, it has been proposed to use oriented polypropylene adhered by adhesive lamination to polyethylene. The polyethylene being heat-sealable allows the film to be formed into a package. Likewise, it has been proposed to coextrude unoriented polypropylene and very low density polyethylene (VLDPE) and adhesively laminate the multi-layer coextrusion at the VLDPE side to another layer of VLDPE. These types of structures however do not provide enhanced shelf life for vegetables with high respiration requirements; for instance, cole slaw, broccoli and cauliflower.
In particular, these packages do not exhibit sufficiently high or sufficiently consistently high OTR, or seal strength.
It has also been proposed to coextrude butadiene styrene as a core layer between two layers of polyethylene such as very low density polyethylene. However, this structure does not allow for resistant type crimp seals as typically used in the snack food industry. The VLDPE is the temperature-sensitive heat seal layer and, the butadiene styrene, which is more heat resistant, is in the core. Thus, the high temperature heat sealing apparatus contacts the more temperature sensitive VLDPE rather than the butadiene styrene. Accordingly, the seal temperature must be sufficiently low so as to prevent damage to the heat-sealable VLDPE layer contacting the heat-sealing apparatus resulting in a low seal strength seal. In addition, such a structure can only be printed upon its outer surface, rendering the printing exposed to scuffing. The package will therefore lose its label or be visually unappealing. Likewise, polyethylene such as linear low density polyethylene has been used as a skin layer in an A/B/A film structure with another material such as a copolymer of ethylene and an ethylenically unsaturated comonomer, for instance ethylene vinyl acetate (EVA), being the core layer. This type of film structure can suffer from the disadvantages of low seal strength because both outer layers are temperature sensitive heat seal layers, and unpleasant odor or taste from the EVA.
However, characteristics such as heat sealability are not the only criteria by which one evaluates a film structure for suitability to package food such as vegetables. Health and safety considerations are very important. Thus, to obtain the desired OTR, certain films are adhered to a monolayer of polyethylene such as ultra linear low density polyethylene (ULDPE). However, films from these resins tend to be soft and cloudy, rendering the film unsuitable with respect to appearance and limiting its utility to impulse seals and not resistant seals.
Typically, polyethylene resins employed in packaging are low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), VLDPE or ULDPE. These polyethylenes are typically made with traditional Ziegler-type catalysts which contain different types of reaction sites which result in the polyethylene containing a broad range of molecules. For instance, the typical polyethylene contains polymers having medium molecular weight with medium branching, low molecular weight with high branching, and high molecular weight with low branching. This results in inconsistent properties.
With respect to packaging materials, ecological considerations as well as economic considerations make it desirable to have as few layers as possible with the most favorable combination of properties. In particular, it would be desirable to have a two- or three- layered structure which has optimal clarity, optimal strength, can be readily heat sealed, has a strong heat seal, can be readily printed upon, has protection for the printing, and, provides the proper barrier properties for products such as vegetables.